A rod sealing system, which comprises a buffer ring, a rod seal, and a dust seal, provided successively in this order from a hydraulic fluid side toward the outside in an annular clearance between two members in reciprocating motion is disclosed in the following Patent Literature 1, and shown in FIG. 1 as a half-cutaway cross-sectional view, where a rod sealing system 1 comprises a rod seal R as the main seal for preventing leakage of a working oil to the outside, a buffer ring B provided on the oil hydraulic side O of the rod seal R to buffer the impact pressure or pressure fluctuations at a high-load stages, or to shut off the inflow of high-temperature working oil into the rod seal R, thereby improving the durability of rod seal R, and a dust seal D for preventing inflow of external muddy water or dusts into the rod seal R at the outside A, as provided in an arrangement in the order of B-R-D from the hydraulic fluid side toward the outside in an annular clearance between two members in reciprocating motion, for example, a rod and a cylinder.
Patent Literature 1: JP-A-2001-355739
To make the longer service life of such a rod seal ring system, specific materials are selected for each seal type of rod seal R, buffer ring B, and dust seal D from the viewpoints of material characteristics such as heat resistance, oil resistance, abrasion resistance, etc. Above all, the buffer ring B is provided on the nearest side to hydraulic oil, and exposed to a high-pressure, high-temperature oil, and thus materials with distinguished strength and heat resistance must be selected for use.
To meet the recent need for higher pressure (35 MPa 42 MPa) and higher temperature (maximum 110° C.), and furthermore longer service life, the afore-mentioned Patent Literature 1 discloses selection of a lip seal type for buffer ring B and use of heat-resistant type polyurethane (polyurethane obtained from heat-resistant type polyol and heat-resistant type polyisocyanate) having distinguished heat resistance and abrasion resistance as the materials. As to rod seal R, it is disclosed to use a lip type and NBR or hydrogenated NBR having distinguished heat resistance, cold resistance, oil resistance, and follow-up to eccentricity as the materials. As to dust seal D, it is disclosed to use a split groove type lip seal with a ring bonded to the outer periphery, and general-purpose polyurethane as the materials from the viewpoint of cold resistance.
In the service temperature up to maximum 110° C., the service life of buffer ring B can be thereby prolonged to attain a load control effect on rod seal R, and thus the prolonged service life of rod sealing system can be obtained. Nowadays, however, there are needs for higher service temperatures up to maximum 120° C. or requirements for further improvement of system service life. When the rod sealing system of the afore-mentioned Patent Literature 1 is used, for example, at a service temperature of 120° C., such a problem as considerable decrease in the sealability has been encountered.
Shapes of members of the rod sealing system will be explained below, referring to FIG. 1. Buffer ring B can be used generally by fitting a backup ring 6 to the inner peripheral seal working as a heel of the sliding side of U-shaped packing 2. Rod seal R can be used generally by providing with a flat washer-type backup ring 11 adjacently to the outward side A of U-shaped packing 8. Dust seal D is fitted to a mounting groove 16 on the inner periphery of cylinder 102, where both of oil lip 13 provided on the oil hydraulic side O and dust lip 14 provided on the outward side A are allowed to slide along rod 104, and a metallic ring 15 is stoving fitted to the outer peripheral engaging part mounting the fitting groove 16.
Numeral 3 shows an inner peripheral lip, 3a a tip end recess thereof, 4 an outer peripheral lip, 5 an inner peripheral heel, 7 a mounting groove, 9 an inner peripheral lip, 9a a tip end recess thereof, 10 an outer peripheral lip, 10a a tip end recess thereof, and 12 a mounting groove, respectively.
Various causes of oil leakage can be presumed. In the sealing system, the rod seal is regarded as an important seal from the viewpoint of oil leakage, and thus the buffer ring is provided in front of the rod seal to buffer a pressure load impact onto the rod seal.
However, due to a loss of resilience caused by a decrease in mechanical strength or due to reduction in interference caused by an increase in abrasion, under conditions in excess of the service limit temperature or in continuous working under such conditions, buffer ring B will fail to function fully (see Paragraph [0022] of afore-mentioned Patent Literature 1). Thus, so long as the buffer ring can work without any damage, longer maintenance of interference will be more effective for pressure load-buffering on the rod seal for a long time. To satisfy the recent need for a higher service environmental temperature (120° C.) and also the need for longer service life, materials having distinguished heat resistance and mechanical characteristics are now required for buffer ring B.
However, the heat resistance and mechanical characteristics of thermoplastic polyurethane for use in the buffer ring B depend on physical restrictions imposed by hydrogen bonds, etc. of hard segments in the polymer chain, and thus have been so far not always satisfactory under the influence of softening or melting point, or heat stability of the hard segment. To improve the heat resistance, or compression set characteristics, it is known to increase the hard segment content. In that case, there is such a problem as an increase in the hardness of molding product resulting in loss of softness.
Attempts have been so far made to replace the components of the hard segment with more rigid ones to improve the heat resistance or compression set characteristics, but have been found not satisfactory. For example, Patent Literature 2 or 3 discloses use of 4,4′-biphenyldiol compounds having a molecule structure with good symmetry and rigidity, as a chain lengthening agent, where the resulting thermoplastic polyurethane has good mechanical characteristics, but suffers from a large reduction in the modulus of elasticity due to increased temperature, and also from an unsatisfactory heat resistance. When p-phenylene diisocyanate as disclosed in Patent Literature 4 is used as a diisocyanate compound, thermoplastic elastomers having distinguished thermal deformation resistance and compression set characteristics can be obtained, but the diisocyanate compound is hard to handle because of a high melting point and also sublimation property, so there is such a problem as thermoplastic elastomers of desired constant quality are hard to obtain.
Patent Literature 2: JP-A-4-211033
Patent Literature 3: JP-A-4-332716
Patent Literature 4: JP-A-1-95119
Thus, it is now difficult to satisfy the mechanical characteristics, heat resistance and compression set characters at the same time by changing the hard segment content or compression ratio of raw material polyurethane. It is known to solve these problems by adding a compounding component, i.e. by adding inorganic fibers such as glass fibers, carbon fibers, etc. or inorganic powder such as calcium carbonate, mica, talc, titanium oxide, whisker, etc. thereto as a reinforcing agent. To obtain a satisfactory effect, however, it is necessary to add a relatively large amount, for example, about 10 to about 30 parts by weight, or more, of the reinforcing agent to 100 parts by weight of thermoplastic polyurethane, sometimes giving rise to such problems as decreases in the rubber elasticity, rigidity, surface flatness, and increases in the specific gravity, etc., of the molding products
On the other hand, Patent Literature 5 discloses another method not based on changes in component composition or compounding condition, that is, elimination of isocyanate groups NCO groups in a certain range (IR absorbance ratio of NCO group/C═C group is 0.10-0.35) remaining in the molding product just after hot molding by heat aging treatment, thereby increasing the cross-linking density to attain chemical stabilization and improve the compression set characteristics, etc. However, the heat resistance of thermoplastic polyurethane is largely dependent also on physical structure such a molecule arrangement (orderly arrangement of hard phases for improving the packing), besides the chemical structures such as cross-linking structure, and thus it is hard to obtain physical stabilities such compression set characteristics only by the method disclosed in Patent Literature 5.
Patent Literature 5: JP-A-7-228661